1. Field of the Invention
This invention relates to improvements in a disposable throwaway cutting tip used for machining metal work.
2. Description of the Prior Art
As is known in the art, in machine tools such as lathes, throwaway cutting tips, triangular or quadrangular in plan view, made from hard alloys such as cemented carbide, are used for an improved operation or for reducing cost. FIGS. 15 to 17 are views showing an example of this type of throwaway cutting tip. In this throwaway cutting tip 1, straight cutting edges 2 are formed between nose portions 3. Each cutting edge 2 has an inwardly formed land portion 4 extending parallel to the cutting edge 2. The land portion has an inwardly formed rake surface 5. Further, inwardly of the rake surface 5 there is an attaching surface 6 provided with a central hole 7 for attaching the throwaway tip to a machine tool.
Generally, a throwaway tip having a construction most suitable for the material to be cut, must be selected and used. However, the throwaway tip 1 has straight cutting edges 2, and negative tips in general have negative rake angles. Thus, as is known in the art, the cutting resistance of throwaway tips is high, posing a problem that chips tend to thermally stick to the cutting edge 2. For example, in the case of cutting a difficultly machinable material, such as stainless steel or Inconel (R.T.M.), the material hardly softens and is tenacious, so that chips thermally stick to the tip to a greater degree and hence the cutting resistance is particularly high. Furthermore, since the work hardening is very high, the cutting resistance further increases, leading to such undesirable phenomena as chipping.
U.S. Pat. No. 3,399,442 (Jones et al) discloses a throwaway tip which has solved the aforesaid problems encountered when a difficultly machinable material such as stainless steel or Inconel (R.T.M.) is cut. Jones et al, as shown in present FIGS. 18 and 19, disclose a throwaway tip 10 which does not have any land. Further, as shown in FIG. 18, the cutting edges 12 are straight as seen in the plan view but in a side view, as shown in FIG. 19, the cutting edges 12 are concave with respect to the attaching surface 16. Thus, the cutting resistance is reduced and thermal sticking of the chips is prevented. Therefore, the tip of Jones et al is widely used for cutting the aforesaid difficultly machinable materials.
However, since the known throwaway tip 10 has concave cutting edges 12, as seen in the side view, the cutting force tends to be concentrated locally. Further, the nose portions 3 which are inherently low in strength, are formed such that the rake surfaces 15 have a predetermined angle. Therefore, the strength of the nose portions 3 is low and hence the cutting areas have an acute angle, posing a problem that they tend to chip. Each concave rake surface 15 has a first surface portion 15a and a second surface portion 15b which functions as a chip breaker, said first surface 15a being gently curved as a whole. Therefore, as shown in FIGS. 20 and 21, a chip 19 produced during the cutting of a work piece 18 comes in contact over a certain width with the wall of the chip breaker surface 15b. Thus, the resistance of contact of the chip 19 on the chip breaker surface 15b is high, resulting in a problem that pressure sticking of the chip 19 occurs or the throwaway tip 10 tends to break.